The proposed study is a secondary analysis of observed in-session clinical processes and their association with treatment outcomes among substance abusing adolescents participating in cognitive- behavioral group therapy. The specific aims of the proposed study are as follows. 1. Observe and code moment-by-moment behaviors occurring during videotaped sessions of adolescent group therapy conducted during a completed randomized clinical trial of adolescent substance abuse treatment. 2. For each adolescent participant, construct a set of two-way contingency tables summarizing critical adolescent-peer and adolescent-therapist behavioral exchanges or sequences occurring within and across group therapy sessions. 2a. Construct four contingency tables for each adolescent group therapy participant summarizing the transitional probabilities of specific observed sequences of adolescent- therapist and adolescent-peer behavioral interaction. 3. Analyze the observational sequential data using Kenny and colleagues' actor-partner independence model adapted for small groups (Kenny et al., 2002). 3a. Use the adolescent-therapist and adolescent-peer contingency table transitional probabilities as dependent variables in actor-partner independence growth curve models for small groups. 3b. Identify changes in adolescent-therapist and adolescent-peer behavioral sequences over the course of group treatment that differ between clusters of adolescents showing clinical improvement versus non-improvement controlling for adolescent- and group- specific sources of variation. The results of the proposed analysis are intended to illuminate clinical processes that characterize and distinguish between effective and ineffective adolescent treatment groups. Importantly, the actor-partner independence model can effectively tease apart adolescent-specific versus group-specific (i.e., peer contagion) effects. This parsing of adolescent- and group-level sources of influence can directly address the questions of whether and to what extent peer contagion effects impact clinical outcomes in adolescent group interventions. Furthermore, the proposed analysis will pinpoint specific types of observed behavioral exchanges through which peer contagion effects may influence adolescent drug use outcomes. Information along these lines can inform clinical guidelines for facilitating positive clinical outcomes in adolescent group treatment.